In ring-structured systems interconnecting a plurality of stations, especially in serial loop communication systems, it is important that each station is synchronized to the stream of data travelling around the loop. This adaptation of station timing to the data stream is necessary to enable correct recognition of data bytes and words by each station. In view of different propagation delays on the various sections of the loop, and varying delays in the station due to processing and/or buffering, and because of insertion or removal of stations into or from the loop, timing relations cannot be predetermined. Thus, it is necessary to initialize a loop system until each station operates in proper timing, or to reinitialize it after a malfunctioning that caused loss of synchronization in several or all stations.
In loop communication systems which include a central station or controller, this station can control initialization by sending commands to the other stations and monitoring resulting answers or conditions on the loop.
In peer-to-peer systems, however, i.e. in systems in which all stations basically have the same functions and no supervisor or control station is provided, specific methods are necessary for achieving proper loop initialization. This involves, according to the present invention, selection of a temporary master station, adapting timing relations in this and all other stations, and initiative by each active station to start an initialization procedure under certain conditions.
In prior art, several loop-type communication systems are known including various methods and means for station selection, timing signals adaptation and station synchronization.
U.S. Pat. No. 3,680,053 to Cotton et al. discloses a ring-structured transmission link which has a plurality of data handling devices connected thereto. A different priority number is allocated to each device. When a device wishes to transmit data it first transmits its own priority number on the ring and the device having the highest priority number of the devices which wish to transmit is allowed to seize the transmission link.
The disclosed method only selects a single station for transmission in an operating loop, while all other stations having a service demand must repeat their requests independently. It is desirable to have a system in which, in a non-synchronized loop, stations cooperate to select a temporary master and to get the whole loop with all active stations into synchronous operation.
U.S. Pat. Nos. 3,919,484 to Maxemchuk and 4,071,706 to Warren both describe loop communication systems in which an adaptive delay function is provided for achieving a total delay around the loop which is an integer multiple of the frame length or packet length used on the loop. In both systems this delay function is provided once only in a loop controller or a single separate synchronizing buffer, respectively. Thus, the used method would not be suitable for a peer-to-peer system in which no single common control element is provided.
In U.S. Pat. Nos. 4,076,964 and 4,086,437, both to Henrion et al., time division multiplex data transmission systems are disclosed in which an adaptation is effected between a received clock rate used for storing into a memory, and a station internal clock rate used for reading from the memory. Adaptation of incoming and outgoing data rates for automatic starting of a communications loop without central control is, however, not taught by these disclosures.
U.S. Pat. No. 4,002,842 to Meyr et al. describes a loop communication system comprising a plurality of subscriber stations and one monitoring station. This monitoring station issues synchronizing bytes to enable synchronizing of bit and byte counters in the subscriber stations, and it supervises the incoming data stream to ensure that sync bytes are generated at appropriate intervals. This system would not be operable if the monitoring station fails. It is desirable to have a mechanism in which any station can be selected as temporary master for starting the loop operation, so that stations may be connected or disconnected as desired.
In U.S. Pat. No. 3,564,145 to Deutsch et al. a loop communication system is disclosed in which a terminal detecting a fault condition because of receiving no data or mutilated data automatically generates a unique terminal address and transmits it over the loop. While downstream terminals are satisfied by these address data, the central controller receiving it can determine the fault location from the received terminal address which is accompanied by an error indication. This fault location detection method requires, however, that a predetermined central controller evaluates the incoming address, and requires that terminals between loop failure and controller still operate in synchronism.
While these prior art patents teach several aspects of selecting, timing and synchronizing in loop or Time Division Multiplexor TDM data communication systems, they do not disclose a complete mechanism for automatic loop initialization and synchronization in a peer-to-peer ring-structured arrangement of stations.
One object of the present invention is therefore a serial loop communication system in which peer-type stations cooperate to initialize operation of the loop and modems so that data transmission between stations is enabled.
Another object of the invention is a serial loop communication system in which automatic loop initialization including adaptation of total loop delay is possible while an arbitrary subset of the system peer stations is active while other stations are disabled or inactive or disconnected.
A further object is a loop system in which each station at which a complete loss of synchronization is detected will start a reinitialization procedure among the active stations connected to the loop.
One more object of the invention is a serial loop fault locating mechanism that allows automatic determination of the location of a fault on the loop which prevents an initialization of the loop operation.
These and other objects are achieved by the invention of which a preferred embodiment is described herein as illustrated in the drawings.
A major advantage of the disclosed system is the possibility to start or restart loop operation with any subset of all possible stations, and the avoidance of a central or monitoring station for loop initialization. Both facts improve the reliability and availability of the peer-to-peer station loop communication system.